1. Field of the Invention
The present invention relates to a lens antenna, particularly to the lens antenna for transmitting/receiving microwave band signals or millimeter-wave band signals and to a method of controlling sidelobe levels.
2. Description of the Related Art
In a conventional lens antenna, a dielectric circular lens is set in an aperture of a horn antenna for the microwave band signals or the millimeter-wave band signals to improve antenna efficiency as disclosed in the official gazette of Japanese Patent Laid-Open No. 219802/1983.
In FIG. 6, symbol 30 denotes a conical horn, 34 denotes a lens, 36 denotes a screw, and 37 denotes a wave absorber. The dielectric lens 34 is circular and is set in the aperture of the metallic conical horn 30. Moreover, in this conventional lens antenna, the wave absorber 37 is bonded to an inner wall of the conical horn 30 with an adhesive to reduce the sidelobe level of the radiation pattern of the lens antenna.
The first problem of the conventional lens antenna lies in the fact that the reflections of high-frequency signals on the lens surface degrade the radiation pattern and antenna efficiency. This is because reflections of high-frequency signals on the lens surface repeat multiple reflections between a surface of the lens and the inner wall of the horn to disturb the power distribution of the high-frequency at the aperture of the lens.
The second problem lies in the fact that, when the wave absorber to the inner wall of the horn is bonded to reduce the sidelobe level of the radiation pattern, high-frequency signals are screened by the wave absorber and antenna efficiency is degraded.
The third problem lies in the fact that the bonding of the wave absorber onto the curved surface of the inner wall of the horn with an adhesive is difficult and reduces productivity.